In recent years, the management of cutter bees (Megachilidae family, Megachile rotundata) has achieved commerical significance due to the importance of the leafcutter bee in pollination of alfalfa in the western United States.
The leafcutter bees are susceptible to a contagious disease known as chalkbrood, caused by the fungus Ascosphaera sp., which spreads by microscopic spores. The disease has significantly reduced the population of leafcutter bees in certain areas, thus resulting in decreased production and economic losses in the alfalfa seed industry.
Chalkbrood was first identified from populations of Megachile rotundata in California in 1973. It has since been recorded from leafcutting bee populations in all states from the Great Plains to the Pacific Coast and all western Provinces of Canada except British Columbia. Not all populations in these areas are diseased but there are few disease-free populations remaining in North America and the incidence of the disease varies greatly from area to area.
The chalkbrood disease turns the body of an infected bee larva into fungal cells which eventually produce millions of spores. The destroyed larvae remaining in the nesting holes is primarily responsible for the transmission of the disease.
Once the disease becomes established in an area it increases rapidly because of the reuse of contaminated nesting media in successive years. Its rapid spread throughout western America is associated with the widespread sale of chalkbrood-contaminated boards and/or the use of such boards for trapping of endemic bee populations in disease-free areas. The disease is spread through local populations by adult bees emerging from contaminated media. It has been shown that a single adult bee may carry from 50 to 300 million spores on its body surface after having chewed through a single diseased cadaver as it extracts itself from the cell or nesting material.
Over the past several years various control measures have been developed but none have been completely effective or economically practical. Sterilization of nesting media has been attempted by use of dry chlorine or bleach, convection heat, and microwave exposure. Also, surface sterilization of adult bees consisting of a bee bath in sodium hypochlorite or iodine has been employed. Dusting with general antibiotics and fungicides in such a manner that they are ingested by the adult bee also has met with little success.
Those concerned with the control of chalkbrood disease in leafcutter bees recognize the need for an improved method to effectively and economically control the disease.